National administration across the world allocate the usable radio frequency spectrum by splitting it into channels of fixed bandwidth. In Europe at VHF and UHF these allocated channels are typically 12.5 kHz wide.
Data is normally sent across these channels in a serial manner, each transmission using the whole channel. The desired data rate and the nominal tolerance of easily-producible frequency references (in the region of 10 parts per million) mean that such systems normally resolve the data over a relatively wide bandwidth (for example 7.5 kHz) which is a large proportion of the allocated channel bandwidth (12.5 kHz).
The fundamental sensitivity of such systems is limited by these bandwidths, the noise figure of the receiver and also any extraneous noise (natural or man-made) present at the receiver.
Many communications systems are limited by relatively low power availability. For example, telemetry applications such as the monitoring of gas meters are power-restricted in that, firstly the remote gas meters have no integral power sources of their own (therefore power must be supplied to them) and, secondly, safety regulations restrict the amount of power that is supplied in the vicinity of the potentially-dangerous gas supply.
For maximum communications range at low power, it is necessary to operate at a low bandwidth. (This reduces the speed at which information can be transmitted, but for many applications, this reduction is acceptable). Consequently, this requires that narrow band filtering is employed, with the wanted signals arriving in this narrow band. In order to achieve this, both the transmitter and receiver must operate with high frequency accuracy else the wanted signal will not be detected by the receiver.
This is a significant problem in communications systems (for example vehicle tracking systems) where there is a need for low cost transmitter units. The high accuracy components required would make the costs prohibitively high.
In our co-pending UK patent application entitled "DIGITAL COMMUNICATIONS" and filed on May 31, 1996, there is provided a digital communications system which seeks to increase the range of a transmitter without increasing its size or cost.
This is achieved by improving the sensitivity of the receiver (ie, enabling it to receive signals at a low signal strength) by implementing said receiver as a bank of narrow filters. Each filter is, effectively, an independent receiver listening to a part of the overall (12.5 kHz) channel. As such, noise power in each filter is reduced and sensitivity therefore increased.
In many applications, more sophisticated communication procedures could be employed if two-way point to multipoint communication were possible. However, although the sensitivity of the receiver at the base station (the "point") is improved using the above method, the same method would need to be employed in every individual outstation (the "multipoints"), again making the costs prohibitively high. Furthermore, there is a problem when considering a transmission from the base site to be received at an outstation because of the disparity in power available between the base station and each outstation, if path of equal range performance is required. As described above, range can be extended by reducing the bandwidth and this is achieved in the base station by the bank of narrow filters. However, for two-way communication where each path needs to operate at a similar budget of power and range, the outstation receiver must also be of narrow bandwidth. The low tolerance of the frequency reference in the desired low-cost multiple outstation is thus a problem again.
It is therefore an object of the present invention to provide a communications system optimised for low cost, power-efficient two-way communications with maximised range between a single base site and a multiplicity of low-cost outstations.